Systems for drilling into geological structures so as to extract oil and gas via a borehole are well known. Particular systems use fluid jets which may be loaded with abrasive particles to erode the rock substrate and drill the required borehole. In such systems typically a rotating drill bit with outlet nozzles is supplied with fluid via a drill string from the surface. Pipe is used to connect the drill bit to the surface and to enable rotation. The pressure of the fluid is such that as the jet of fluid issues from the nozzles, the force of the impact is expended in the substrate so causing removal of the rock substrate to create a borehole. Typically drilling occurs through a number of different substrates or strata until the appropriate stratum containing the substance of interest is reached. The substance may be oil or gas.
The nozzles placed in the drill bit are typically spaced about the bottom area of the drill bit at varying radii in order to give the desired cutting profile into the rock substrate. Such a system is described in U.S. Pat. No. 3,924,698 where a number of nozzles are positioned to drill grooves into the rock substrate below the drill bit. The nozzles are arranged at varying angles depending on their position from the central axis of the drill bit so as to ensure that the grooves drilled are of equal depth. These systems are suitable for direct drilling down through a substrate where a high pressure is available at the surface and can be substantially maintained down to the drill bit and utilised in jetting fluid through the nozzles.
In the system described in U.S. Pat. No. 3,924,698, during operation the nozzles in the rotating drill bit expel a stream of abrasive laden fluid to act against the rock bed. As the drill bit moves further down the borehole, stand off elements on the bottom of the bit are forced against the partially eroded rock structure and readily break down the walls between the individual grooves.
In existing boreholes that have been used to tap strata bearing a particular substance, the extraction rate of the substance from the borehole decreases with time. To increase the extraction rate the channel from the stratum that is tapped by the borehole must be opened up. Typically this involves casing window milling and using a drill string to drill a lateral well. However when the target stratum is thin, such systems are not suitable.
Attempts have been made to use abrasive free cutting systems with more flexible coiled tube(CT) systems to drill lateral drain holes into the stratum to improve the extraction rate of the substance contained in the stratum. However these known systems suffer severe performance problems and the pressure drop at the nozzle of 10,000 psi is such that it is unsuitable for use with CT systems in downhole conditions.
A further coiled tubing system is described in U.S. Pat. No. 5,413,184 where a ball cutter is coupled to the tubing and lowered into the borehole. The ball cutter cuts through the borehole casing which limits the exposure of the borehole to the substance bearing stratum and is moved outwardly into the stratum for a preselected distance. After the preselected distance is reached, the ball cutter and tubing are wound back to the surface and the ball cutter replaced with a nozzle blaster. The nozzle blaster is lowered into the borehole until it extends through the channel previously created by the ball cutter. When the end of the pre-cut channel is reached, fluid is pumped through the nozzle blaster to cut through the stratum. This system requires extraction of the tubing between the successive cutting stages and the direction of travel of the nozzle blaster is determined by the pre-cut channel.